1. Field of the Invention
The present invention relates to mobile communication systems and more particularly to a method and apparatus for performing a random access procedure in a mobile communication system.
2. Description of the Related Art
In Long Term Evolution (LTE) whose standardization work is being conducted in 3rd Generation Partnership Project (3GPP) as one of the next generation mobile communication systems, an Evolved Node B (ENB) takes charge of scheduling downlink (DL) and uplink (UL) traffics. In the communication system where the ENB takes charge of the scheduling, a procedure for transmitting UL data includes the following five steps.
Step 1: A User Equipment (UE) transmits to an ENB a signal indicating generation of UL data.
Step 2: The ENB allocates transmission resources to the UE so that it can report information on the generated data, for example, the amount or priority of the data.
Step 3: The UE sends to the ENB a Buffer Status Report (BSR) message in which the amount and priority information of the data stored in its buffer is contained.
Step 4: Based on the BSR message, the ENB allocates transmission resources necessary for actual data transmission to the UE.
Step 5: The UE transmits data using the allocated transmission resources.
With reference to FIG. 1A, a description will now be made of a process in which a UE makes a buffer status report by sending a scheduling request message to an ENB in an LTE system.
The message that the UE sends in Step 1 is referred to as a scheduling request message, the resources over which the scheduling request message are called scheduling request transmission resources (or scheduling request resources), and the periodically repeated scheduling request resources can be allocated to a UE in the state where its Radio Resource Control (RRC) connection is set up. For example, particular transmission resources can be given to a particular UE as scheduling request transmission resources every 10 msec. When there is a need for data transmission over UL, a UE 105 that has the periodically-existing scheduling request transmission resources sends a scheduling request message to an ENB 110 through the scheduling request resources in step 115.
The scheduling request message can be expressed with a single bit of (1-bit) information, and the UE 105 requests transmission resource allocation for buffer status report using the 1-bit information. In step 120, the ENB 110 sends a UL grant message to allocate to the UE 105 as many transmission resources as the UE 105 may send in a BSR message for a buffer status report. In step 125, the UE 105 sends the BSR message containing the type and size of the generated data to the ENB 110 through the transmission resources allocated in step 120 through the UL grant message.
Based on the BSR message, the ENB 110 estimates priority of the data, and allocates transmission resources to the UE 105 at an appropriate time so that it can transmit UL data, taking into account both the channel state of the UE 105 and the traffic condition of the current cell.
FIG. 1B is a diagram illustrating an example of a BSR message used in an LTE system.
The BSR message, since it is a Medium Access Control (MAC) control message made by a MAC layer, is a kind of a MAC control Service Data Unit (SDU), and is contained in a normal MAC Packet Data Unit (PDU) 140 before being transmitted. The MAC PDU 140 is generated with a MAC header 130 and a MAC SDU 135. The MAC header 130 contains the type and size of the MAC SDU 135. The terms ‘PDU’ and ‘SDU’ refer to the data that underwent a particular operation in an arbitrary protocol device, and when the data generated in an upper layer is delivered to an arbitrary protocol device, the upper layer data is called ‘SDU’. Similarly, when the protocol device generates its own control information, the control information is also called ‘SDU’. However, the SDU, to which a predetermined protocol header is added, is called ‘PDU’.
Since the ENB cannot allocate scheduling request resources to all UEs to which it has RRC connections, the ENB allocates no scheduling request resource to the UEs having a lower frequency of data transmissions. Since the only UL channel that the UE can use, which is not allocated scheduling request resource evens though its RRC connection has been set up, is a random access channel, the UE which is not allocated scheduling request resource sends a BSR message to the ENB through a random access procedure. However, no definition has been given of a method in which the UE in the state where its RRC connection is set up sends a BSR message to the ENB through the random access procedure.